The invention concerns circuitry for demagnetizing the mask in a color picture tube by means of a demagnetizing coil mounted thereon with a series-connected resistor that has a positive temperature-coefficient (PTC) applied to the incoming alternating current when the receiver that includes the picture tube is turned on.
Known systems have a PTC resistor that is thermally connected to a similar resistor. These PTC resistors make it possible to continuously decrease over time the alternating current traveling through the demagnetizing coil to a residual current dictated by the characteristic of the resistor. The residual current can be discontinued by means of such components as a relay or TRIAC in series with the demagnetizing coil. The current is discontinued suddenly, leaving a residual field that can disrupt the uniformity of the black-and-white compensation. Aside from the expensive components it demands, this system has an additional drawback in that the PTC resistor cools off once the demagnetizing current has been discontinued and allows thereby a parameter that corresponds to the horizontal-frequency current to be induced in the deflection coil, which can also cause malfunction in the color unit.